[PATCH mm-hotfixes] mm/zswap: add missing kunmap_local()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

Commit e2c3b6b21c77 ("mm: zswap: use SG list decompression APIs from
zsmalloc") updated zswap_decompress() to use the scatterwalk API to copy
data for uncompressed pages.

In doing so, it mapped kernel memory locally for 32-bit kernels using
kmap_local_folio(), however it never unmapped this memory.

This resulted in the linked syzbot report where a BUG_ON() is triggered due
to leaking the kmap slot.

This patch fixes the issue by explicitly unmapping the established kmap.

Reported-by: syzbot+fe426bef95363177631d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69b75e2c.050a0220.12d28.015a.GAE@google.com
Fixes: e2c3b6b21c77 ("mm: zswap: use SG list decompression APIs from zsmalloc")
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 mm/zswap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index e6ec3295bdb0..499520f65ff0 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -942,9 +942,14 @@ static bool zswap_decompress(struct zswap_entry *entry, struct folio *folio)

 	/* zswap entries of length PAGE_SIZE are not compressed. */
 	if (entry->length == PAGE_SIZE) {
+		void *dst;
+
 		WARN_ON_ONCE(input->length != PAGE_SIZE);
-		memcpy_from_sglist(kmap_local_folio(folio, 0), input, 0, PAGE_SIZE);
+
+		dst = kmap_local_folio(folio, 0);
+		memcpy_from_sglist(dst, input, 0, PAGE_SIZE);
 		dlen = PAGE_SIZE;
+		kunmap_local(dst);
 	} else {
 		sg_init_table(&output, 1);
 		sg_set_folio(&output, folio, PAGE_SIZE, 0);
--
2.53.0